Metal-Organic Frameworks (MOFs) have garnered significant interests in the last two decades due to their promising potential in many applications such as gas adsorption, separation, catalysis and sensing. For example, see Yaghi, O. M.; O'Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705. (b) Ferey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F. Acc. Chem. Res. 2005, 38, 217. (c) Horike, S.; Shimomura, S.; Kitagawa, S. Nat. Chem. 2009, 1, 695. (d) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim, K. Nature 2000, 404, 982. (e) Jiang, H.-L.; Liu, B.; Akita, T.; Haruta, M; Sakurai, H.; Xu, Q. J. Am. Chem. Soc. 2009, 131, 11302. (f) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Chem. Rev. 2012, 112, 1105. (g) Yang, S.; Liu, L.; Sun, J.; Thomas, K. M.; Davies, A. J.; George, M. W.; Blake, A. J.; Hill, A. H.; Fitch, A. N.; Tang, C. C.; Schröder, M. J. Am. Chem. Soc. 2013, 135, 4954. (h) Bloch, E. D.; Queen, W. L.; Krishna, R.; Zadrozny, J. M.; Brown, C. M.; Long, J. R. Science 2012, 335, 1606. (i) Wang, Z.; Cohen, S. M. Chem. Soc. Rev. 2009, 38, 1315.
Compared with other porous materials such as zeolite and mesoporous silica, MOFs are based on crystalline porous structures tunable on the atomic scale, which can be designed and functionalized by judicious choice of metal nodes and modification of the organic linkers. However, one of the limitations of most MOFs is their low chemical stability, which undoubtedly hampers their application in industry. A rule of thumb for the construction of stable MOFs comes from the simple Hard and Soft Acid and Base Theory, which guides the selection of the metal-ligand combination for a MOF. For example, see Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533. Because the carboxylate group is a hard Lewis base, hard Lewis acids such as Fe3+, Cr3+, Zr4+ and Ti4+ are usually considered good candidates for the construction of robust MOFs. This method has become the focus of some recent research efforts but very few stable MOFs have been obtained, especially in single crystal form. For example, see (a) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. J. Am. Chem. Soc. 2008, 130, 13850. (b) Ferey, G.; Serre, C. Chem. Soc. Rev. 2009, 38, 1380. (c) Phan, A.; Doonan, C. J.; Uribe-Romo, F. J.; Knobler, C. B.; O'Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2010, 43, 58. (d). Murray, L. J.; Dinca, M.; Yano, J.; Chavan, S.; Bordiga, S.; Brown, C. M.; Long. J. R. J. Am. Chem. Soc. 2010, 132, 7856. (e) Feng, D.; Gu, Z.-Y.; Li, J.-R.; Jiang, H.-L.; Wei, Z.; Zhou, H.-C. Angew. Chem. Int. Ed. 2012, 51, 10307. (f) Jiang, H.-L.; Feng, D.; Liu, T.-F.; Li, J.-R.; Zhou, H.-C. J. Am. Chem. Soc. 2012, 134, 14690. The main reason is that MOFs based on these metal ions of high valence are difficult to crystallize. Occasionally, MOFs in the form of crystalline powder were obtained, but structure solution and refinement based on Powder X-Ray Diffraction (PXRD) data is not straightforward. Furthermore, the incorporation of rarely reported metal nodes into MOFs is less predictable and controllable.
There is also a need to provide MOFs in monocrystalline and polycrystalline form. A monocrystalline MOF (or a single crystal MOF) consists of a MOF in which the crystal lattice of the entire solid is continuous, unbroken (with no grain boundaries) to its edges. Monocrystalline is opposed to amorphous material, in which the atomic order is limited to short range order only. Polycrystalline materials lie between these two extremes; they are made up of small crystals. They are different from monocrystalline materials. Large single crystals are very rare in nature and can be difficult to produce in the laboratory.
Metal-organic frameworks are coordination polymers having an inorganic-organic hybrid framework that comprises metal ions and organic ligands coordinated to the metal ions. These materials may be three-dimensional, i.e. have three-dimensional lattices in which the metallic species are joined together periodically by spacer ligands.
Metal-organic frameworks have many applications including, for example, in the field of adsorption, storage, separation or controlled release of chemical substances, such as, for example gases, or in the field of catalysis. Metal-organic frameworks may also be useful in the field of pharmaceuticals (controlled release of medicaments) and in the field of cosmetics.
There are in fact a growing number of applications for metal-organic frameworks and as such there is an ever growing need for new such materials with a variety of properties and a need for new metal-organic frameworks having improved properties.
In addition, there is a need to develop new processes for preparing metal-organic frameworks that allow for the preparation of a wide variety of metal-organic frameworks and/or improve the quality of the metal-organic frameworks obtained.
However, metal organic (framework) powder material has been prepared by various methods but prior to the present invention large single crystals of metal organic frameworks containing a number of different metal ions have not been prepared.
For example, a number of iron metal organic frameworks have been synthesised but there remains a need to develop a synthesis that is robust and can be used to prepare a wide range of metal organic frameworks. In particular, there remains a need to provide monocrystalline iron metal organic frameworks having large crystal size. Furthermore, monocrystalline aluminium and titanium metal organic frameworks have not been prepared prior to the present invention. In fact, serious difficulties have been experienced preparing crystalline metal organic frameworks containing iron, aluminium or titanium.
As evidenced in the literature, it is still not possible to provide MOFs containing a wide range of metal ions that have large crystal sizes. In fact, a number of difficulties still exist that prevent successful synthesis of large crystal MOFs. These difficulties are also put into context when the size of crystals achieved in the literature is reviewed.
Firstly, the zinc based molecular organic framework MOF-5 (ZN4O(BDC)3) is one of the first and most successful MOFs reported to date. It is reported, e.g. see Nanomaterials in Catalysis edited by Phillipe Serp, Karine Philippot, that single and polycrystalline cubic crystals of MOF-5 have been prepared with crystal size ranging from 50 to 200 μm. MOF-5 could be viewed as the standard bearer of MOFs against which all subsequent MOFs are compared. In reality, since details of MOF-5 were first made available, no-one has been able to achieve similar crystal sizes to those achieved by MOF-5. In particular, there are no reports of any MOFs containing other metal ions (e.g. iron, aluminium, and titanium) having crystals sizes anywhere near that reported for MOF-5.
For iron MOFs, U.S. Pat. No. 8,173,827 B2 exemplifies the preparation of a couple of MOFs (namely Fe-BTC-1 and Fe-BTC-2). However, the MOFs described in this patent have been prepared only on the nanoscale. For example, Fe-BTC-1 is reported to have a particle size ranging from 0.2 μm to 0.5 μm; and Fe-BTC-2 is reported to have a particle size ranging from 2 μm to 5 μm. These crystal sizes are a factor of 10 smaller than those reported for MOF-5. Furthermore, these MOFs appear to be amorphous materials; the processes described therein fail to provide crystalline materials, let alone monocrystalline materials.
In addition, Horcajada et al in Nature Materials, Vol. 9, February 2010, 172-178 describes the preparation of a number of iron MOFs but the authors have not been successful in preparing large crystals of iron MOFs. In contrast, they have only bee able to prepare iron-MOFs on the nanoscale. For example, MOFs labelled MIL-53, MIL-88, MIL-100 and MIL-101 have been prepared and particle sizes ranging from 50 nm to 350 nm are reported. The authors have therefore failed to prepare a MOF having a particle size greater than 0.35 μm.
In addition, Langmuir 2011, 27, 15261-15267 reports the preparation of an iron MOF labelled Fe-MIL-88B—NH2. However, again the authors have only been able to prepare the iron-MOF with small crystal sizes (e.g. 3.5 μm by 1.2 μm).
For titanium MOFs, the realization of polycrystalline structures remains elusive. In the literature, Kaskel et al (J. Mater. Chem., 2006, 16, 2354-2357) appear to be the first to describe a titanium porous inorganic-organic hybrid material (labelled in the prior art as TT-1). However, the titanium based MOF prepared by Kaskel et al is amorphous; it is not crystalline. The Kaskel et al document itself acknowledges that only a few titania-based MOFs have been prepared because of the difficulties experienced. None of them are crystalline, let alone monocrystalline having a large crystal size. In 2009, Ferey et al (J. Am. Chem. Soc. 2009, 131, 10857-10859) described a “well-crystallised white hybrid solid” (labelled in the prior art MIL-125) which was also the subject of US patent application US2012/0121904 A1. However, the crystals of MIL-125 which have been prepared are relatively small. For example, particle sizes for MIL-125 are reported by Kim et al in Catalysis Today 204 (2013) 85-89 to range from 0.004 mm to 0.005 mm.
For aluminium MOFs, there is very little literature associated with the preparation of aluminium-based MOFs. In the literature, Ferey et al (Chem. Mater. 2009, 21, 5695-5697 & Chem. Mater. 2009, 21, 5783-5791) describe the preparation of aluminium MOFs (labelled as MIL-100, MIL-120, and MIL-121) using hydrothermal synthesis, which were also the subject of US patent application US2012/0055880 A1. The sizes of the crystals obtained were however relatively small. For example, US2012/0055880 reports crystal sizes ranging from 1 micron up to only 30 microns (0.001 mm to 0.03 mm).
There is therefore a need to provide crystalline MOFs that exhibit a greater crystal size. As well as providing crystalline products, there is also a need to provide MOFs in monocrystalline and polycrystalline form. A monocrystalline MOF (or a single crystal MOF) consists of a MOF in which the crystal lattice of the entire solid is continuous, unbroken (with no grain boundaries) to its edges. Monocrystalline is opposed to amorphous material, in which the atomic order is limited to short range order only. Polycrystalline materials lie between these two extremes; they are made up of small crystals. They are different from amorphous materials. Large single crystals are very rare in nature and can be difficult to produce in the laboratory.
Metal-organic frameworks are coordination polymers having an inorganic-organic hybrid framework that comprises metal ions and organic ligands coordinated to the metal ions. These materials may have three-dimensional lattices in which the metallic species are joined together periodically by spacer ligands.
As discussed above, to date it has not been possible to prepare MOFs having large crystal sizes and excellent stability using a process that is robust and suitable for preparing MOFs involving a wide range of metal ions and a wide range of carboxylate ligands.